In the field of personal adornment, colored stones have assumed great significance to the extent that demand today far surpasses supply. In view of the expense and costs involved in obtaining colored gemstones, a search is on for alternatives to colored stones. A vast range of colored as well as colorless minerals used as gemstones are being mined all over the world. The stones so obtained are cut and polished to make them suitable for a specific use. Such precious and semi-precious gemstones are extensively used in jewelry, decorative articles and ornaments. As demand of colored stones is huge compared to their natural availability, colorless and low colored natural gemstones are subjected to variety of treatments to enhance their properties like color, luster or transparency.
Certain colored stones and crystals have been sought and prized for their beauty beginning with ancient civilizations and continuing into the modem age, even to the extent of ascribing to them magical powers. Through appropriate cutting and polishing, the value and natural beauty of a precious stone may be further enhanced. While a vivid color is not always a requirement for a jewel, i.e., cut stone, it is an important attribute of red ruby, blue sapphire or green emerald. Furthermore, the color or lack thereof may be an important attribute of relatively clear gems, such as diamonds.
Besides its visual beauty, a gemstone for ornamental purposes has to possess a certain amount of durability to be classified as precious, since it should be displayable by the owner without fear of deterioration or tarnish. Gemstones of the requisite hardness to qualify as durable include gem varieties derived from conundrum (e.g., ruby, and sapphire) and from carbon (e.g., diamond). Such gemstones typically not only outlast their wearers but also sparkle with undiminished luster long after their metal setting has been abraded or eroded.
Finally, naturally occurring gemstones generally are of higher inherent value than artificially grown gems, regardless of the fact that artificial stones often are of purer color and higher internal crystal quality. Although this result is probably attributable to the lack of rarity in the artificial stone, some minor flaws in naturally occurring stones are thought to add beauty and character to the final jewel.
Naturally occurring gemstones may exhibit undesirable color zones in which the color is distributed unevenly in the gem. The uneven color zones generally make it more difficult for the lapidary to select the direction in which the gem should be advantageously cut. On the other hand, colorless crystals often are less attractive than colored ones.
Various attempts have been made to produce jewelry items of inexpensive materials with the appearance of expensive materials. For example, U.S. Pat. No. 1,005,564 describes a technique of employing colored sheets of celluloid to form various shapes to represent various natural materials and gems, such as coral, turquoise and the like. U.S. Pat. No. 4,295,347 describes other techniques employing lacquers on a substrate in order to simulate the appearance of a gem. U.S. Pat. No. 4,835,023 describes the use of various coatings on cultured pearls in order to improve the quality of the pearls. However, to date, the techniques employed to process an inexpensive material to give the appearance of an expensive material have been cumbersome, and, in some cases, have not achieved the desired result.
U.S. Pat. No. 6,146,723 discloses an enhanced gemstone and a method of simulating the appearance of an expensive gemstone. In the process of this invention, a clear crystalline faceted substrate is provided with a coating of colored transparent ink in order to enhance the appearance of the otherwise inexpensive gemstone. The ink is permanent type ink made from n-propanol, n-butanol and diacetone alcohol, which is soluble in a solution having isopropyl alcohol 99% strength. Once the coating has been applied to a substrate such as a cubic zirconia to enhance the appearance of the gemstone, the coating is removed by use of the solvent.
U.S. Pat. No. 5,853,826 discloses a method for the improvement in the color of transparent materials including gemstones. A faceted gemstone having at least one thin layer of material is coated on the pavilion of a transparent substrate so that the body of the gemstone appears to have a different color. The color of reflected light changes hues when viewed through the face of the gemstone at different angles of observation. This causes the gemstone to appear to change color when the stone is tilted. The thin film is coated by low temperature techniques such as sputtering.
A number of processes have been developed to improve the appearance of gemstones or to create simulated gemstones. Diffusion of ions into gemstones, such as titanium and/or iron into sapphire to give a blue color has been disclosed. This process is limited to specific ions and specific substrates. This process requires extremely high temperatures, which frequently damages the substrates. Diffusion processes cause the added ions to become part of the crystal surface with no distinct boundary and usually form a gradient of ion concentration in the base substrate. Examples of diffusion processes include U.S. Pat. Nos. 2,690,630 and 4,039,726.
TAVELITE™ is a product produced by depositing thin multiple layers on a transparent substrate to produce an interference effect. The process utilizes high temperatures and frequently results in substantial breakage of the product.
AQUA-AURA™, a product of Vision Industries is a surface treatment providing a single, moderately saturated, color. This process is proprietary but is reported to be a coating based upon gold and the use of high temperatures.
Nuclear radiation has been used to produce color centers in gemstones, giving a body color that can sometimes be improved with heat treatment. Cyclotrons and neutron bombardment are routinely used to impart blue color to colorless topaz.
Rhinestones and Carnival Glass have reflective coatings layered on one or more surfaces of a clear substrate. The coating is usually silver or some other highly reflective material utilized to apply a mirror coating (usually silver or aluminum) onto the pavilion (back) of a faceted glass gem. In such a coating, all light is reflected without passing through the coating. Atmospheric pressure chemical vapor deposition has been used to deposit films of titanium oxide by the thermal decomposition of a titanium compound (usually TiCl4) in air.
Layered coatings on a surface of gemstones have been done to increase the “fire” of the stone. These techniques involve the coating of a highly refractive material, with respect to the gemstone's index of refraction, followed by a second coating of a different highly refractive material. The layers are designed so that the reflected light at each interface of each layer causes an optical interference effect. An example of such a method is U.S. Pat. No. 3,490,250.
U.S. Pat. No. 5,084,909 describes color enhancement brought about in natural and synthetic gem materials by high-energy gamma ray fields for extended periods of time (50 to 1000 hours). High-energy radiation fields are necessary for this process. It is extremely interesting to note that disclosed in this reference the inventor utilized heat to bleach or remove unwanted colors from the gems (either before or after a well-known color enhancement procedure that involved electron bombardment). Therefore, this reference clearly teaches that heat has been utilized not to enhance color but to lessen color intensity in at least gem-type materials.
U.S. Pat. No. 5,477,055 presents a thorough review of the history of coloring precious or semi-precious gemstones via single or combined radiation treatments. The color enhancement process related is a two step method that includes fast neutron irradiation at between 350° C. to 600° C. followed by gamma ray or electron bombardment. Higher temperatures tend to fragment the gemstones. The elevated temperatures (above room temperature) tend to reduce unwanted side (blue-gray) colors.
U.S. Pat. No. 4,749,869 discloses a process for irradiating topaz and the product resulting therefrom. A three-step method of color enhancement is described in which a sample stone is: 1) exposed to high energy neutrons; 2) exposed to electrons; and 3) heated to between 250° F. (121° C.) and 900° F. (482° C.). The heating step tends to “bleach-out” or remove unwanted side colors, thereby enhancing the desired blue color.
U.S. Pat. No. 2,945,793 discloses a method for heat treatment of diamonds to increase desirable coloration. Irradiation by electrons in followed by heating to about 500° C. to again, decrease undesirable tints within the diamond. However, this invention relates specifically only to reducing the color of diamonds in order to enhance the clarity and transparency.
U.S. Pat. No. 6,025,060 discloses a method and apparatus for creating unique gemstones. The method comprises the steps of optically contacting the gemstones of interest followed by a heat treatment of the composite gemstone. The heat treatment step increases the bond strength and therefore the resistance of the bond to reversal. In one aspect of the invention, a composite gem is fabricated by bonding a naturally occurring gem to an artificial gem to form a single composite gemstone of large size that outwardly appears to be a single natural gem. The composite gem may be fabricated at a fraction of the cost of a natural stone of the same size. In another aspect of the invention, an intensely colored natural stone is bonded to a colorless or lightly colored artificial stone. This composite retains the intense color associated with the natural stone while enjoying the brilliance, depth, and size resulting from the combination of stones. In another aspect of the invention, various composite gemstones are fabricated using a variety of stones of both natural and artificial origin. The stones may be layered with two, three, or more layers. The composite gem may either take the form of a simple layered gem, or the composite gem may be in the form of a variety of three-dimensional shapes. In another aspect of the invention, the composite gem includes an engraved pattern at one or more internal gem interfaces. The engraving is completed prior to bonding the stones together and may convey either a two-dimensional or a three-dimensional image.
U.S. Pat. No. 4,039,726 discloses changing the color of conundrum by diffusing chromophores such as iron/titanium, chromium, chromium/nickel, or chromium/iron/titanium into natural or artificial crystals at temperatures typically above 1700° C.
In the last decade, a variety of techniques such as electron beam irradiation [U.S. Pat. No. 5,637,878, Herer, Arnold S., Knobel, Thomas M. and Robb, Gregory J., Jun. 10, 1997}, Cobalt-60 irradiation [U.S. Pat. No. 5,084,909, Pollak, January 1992] neutron bombardment [U.S. Pat. No. 4,749,869, Richard Fournier, June 1988], heat treatment, diffusion and the like, have been employed in the art to produce colors in colorless gems and/or enhance the properties of colored stones.
However, the hitherto known processes mentioned above suffer from significant drawbacks in term of cost, safety, efficacy and the like. In irradiation, a widely used commercial process, the stones are exposed to a beam of high-energy subatomic particles or electromagnetic radiation such as neutron or gamma rays either to enhance color or to produce a color in colorless stones. Commercially used radiation includes: high-speed negatively charged electrons from a linear accelerator; high-speed alpha particles (positively charged helium nuclei) from either a linear accelerator or a cyclotron; energetic neutrons from an atomic pile; and high energy gamma rays from a Cobalt-60 source. Charged particles induce color skin depth due to their small size while high-energy neutron and gamma irradiation color the whole stone. The dose and energy of the particle beam determine the quality and degree of color induction in gemstones. Subsequently, the irradiated samples are heat treated to stabilise the color. For example, diamond, zircon, topaz, quartz, conundrum, beryl etc. are subjected to irradiation for enhancing their color and appearance. [R. Webster, ‘GEMS’ Butterworths—Heinemann Ltd Oxford, London, 1994]. Heat treatment is an ancient method to enhance properties of most of the gemstones. Quartz, topaz, zircon, conundrum, ruby, sapphire, amethyst, citrine, aquamarine are examples of gemstones whose colors can be altered by heat treatment (Ted Themelis, “The heat treatment of Ruby and Sapphire” ISBN 0-9409-6510-0, Published by Gemlab. Inc., Type-ergraphics Inc. USA] In recent years, diffusion technique has also been reported to produce color in gemstones. [J. I. Koivula, and R. C. Kammerling, Gems & Gemmology, Vol. 26, No. 1, PPI00-101]. In this method, the gemstones are heated in powder of a metal or a metallic compound at a high temperature to produce colors in them.
While irradiation has been established world over as a commercial technique to enhance and produce color gemstones, particularly for the manufacture of blue Topaz, this technique requires expensive and complex equipment i.e. accelerator to produce an energetic beam of particles. The cost and maintenance of an accelerator is very high. An established safety procedure has to be followed to operate the system and highly skilled operators are needed to run and maintain the equipment. Further, safety demands cooling of the radiated samples as treated stones emit hazardous radiation. Therefore, the irradiated gemstones require storage in a specially designed container for a period of a few weeks to a few months to kill the emission of hazardous radiation from the stones, before these are marketed. Even after this cooling, the samples may emit the reminiscent radiation [Crowningshield, Robert, ‘Irradiated topaz and radioactivity’, Gems and Gemology, 17, No. 4, 1984, pp 179-180].
Another disadvantage in the process of irradiating stones to enhance or impart color is that of permanency. The exposure of radiated stones to high temperature during use on occasion results in the loss of color. Additionally, irradiation technique is a multi-step process. The stones are first subjected to neutron or gamma radiation to induce color, and then subjected to electron exposure for color uniformity and then a heat treatment at a temperature between 150 to 1100° C. for a few hours to stabilize color [U.S. Pat. No. 5,637,878, Herer, Arnold S., Knobel, Thomas M. and Robb, Gregory J., Jun. 10, 1997}, U.S. Pat. No. 5,084,909, Pollak, January 1992, U.S. Pat. No. 4,749,869, Richard Foumier, June 1988],
The disadvantage of heat treatment processes known in the art is that color is induced only in some specific minerals. One group of such minerals is called idiochromatic minerals, which contain one of the transition elements as its constituent. The second group of gemstones is allochromatic gems, which are colored due to the presence of small quantities of various transition elements as impurities [K. Nassau, ‘Physics and Chemistry of color’ Wiley—Interscience Publication, John Wiley & Sons, N.Y, 1983]. The third group owes their hue to a color centre, which is a defect in a crystal, which is able to trap an electron [K., Nassau, ‘The origin of color in minerals and gems.’ Lapidary J. In 3 parts: 29: 920-28, 1060-70, 1250-58, and 1521, 1975]. Therefore, the yield of treatment may not be cost effective unless all stones belong to one of the above categories.
Colouring of minerals by diffusion is achieved through heating the stones with a color—causing compound in powder form. The elements from the powder diffuse into the surface of the stones to induce color in the surface. This powder-based diffusion generally damages the surface resulting in low yield.
Recently, a two-step process has been developed to improve the yield [U.S. Pat. No. 5,888,918, Pollak, Richard, March 1999]. In this process, initially the stones in combination with a treating agent in powder form are heated together followed again by a high temperature treatment at a temperature in the range of 900° up to about 1250° C. for a time in the range of about 3 up to 200 hours in the absence of the treating agent to produce desired color. The use of two separate temperature cycles will result in low yield and is therefore not suitable for economical commercial production. Another disadvantage observed in this two-step treatment process is that uniformity in color is not readily achieved.